The present invention relates to a process for hydrotreating heavy hydrocarbon oils containing asphaltenes and metallic contaminants. In more detail, the present invention relates to a process for treating heavy hydrocarbon oils with hydrogen in the presence of a novel demetallization catalyst and then treating the demetallized hydrocarbon oils in the presence of a novel hydrodesulfurization catalyst or hydrocracking catalyst.
Recently as crude oils have become more heavy, there is a growing tendency that the amounts of asphaltenes and metallic contaminants generally present as organometallic compounds in crude oils and residual oils increase. Also, the metals mainly found in petroleum hydrocarbon oils such as reduced oil and the like and heavy hydrocarbon oils recovered from tar sand, oil shale and the like by a well known dry distillation are nickel, vanadium and iron. These metals are present in heavy oils in a free state. However, considerable amounts of said metals are present in asphaltenes and other high molecular compounds in the form of metallo porphyrin. Asphaltenes and metallic contaminants are very poisonous to hydrocarbon conversion catalysts. When heavy oils containing such poisonous materials are to be hydrodesulfulized or hydrocracked with catalyst, the pores of the catalyst used become clogged and the activity of the catalyst is lost in a short time. Therefore to maintain the activity of the hydrocarbon conversion catalysts at a high level for a long period, previous removal of such metallic contaminants from heavy oils is preferred.
In view of the fact that metallic contaminants in heavy oils are mainly contained in asphaltenes, in this art there has been prevailing the thinking that, as demetallization catalysts, catalysts having a large pore size sufficient for hydrocracking the greater part of the asphaltenes and also having large amounts of active metal components are effective. When this demetallization catalyst is used, the hydrocracking catalyst and hydrodesulfurizing catalysts on the downstream side of the demetallization step are surely guarded from poisoning due to metallic contaminants. But there is an objectionable point that the high desulfurization activity of such demetallization catalyst is lost in a short time due to metal deposit. Accordingly, viewed from the point of the overall hydrotreating process which comprises first demetallizing heavy oils and then hydrocracking or hydrodesulfurizing the same, it is not always recommendable to use the catalysts having a large pore size and containing active metals in large amounts in the demetallization step.
The present invention relates to the improvement of the overall process for hydrotreating heavy oils by using the catalyst having a small pore size and containing active metals in small amounts as compared with the conventional demetallization catalysts in the demetallization step and using the catalyst having a large pore size and containing active metals in large amounts as compared with the conventional hydrodesulfurization catalysts or hydrocracking catalysts in the hydrodesulfurization step or hydrocracking step.
Since asphaltenes in heavy hydrocarbon oils are subjected to thermal dissociation under high temperature and high hydrogen pressure are connected into relatively low molecular weight molecules, the demetallization catalyst having a small pore size can effectively remove the metallic contaminants from the relatively low molecular weight asphaltenes that have already been subjected to thermal dissociation and also the other metallic contaminants rich in reactivity. The metallic contaminants not removed in the demetallization step, as a matter of course, are fed to the hydrodesulfurization step or hydrocracking step. However, as the catalyst used in said step has a relatively large pore size and additionally contains relatively large amounts of active metals, there is little possibility of the hydrodesulfurization or hydrocracking being severely disturbed.